A gate circuit of this type is called "preceding value holding gate circuit" and configured as shown in FIG. 3 in which if pulsive noises N are included in an input signal, a switching circuit is opened for a predetermined switching time T to remove the noises N, and the gate circuit supplies a signal having a level preceding the off-action of the switching circuit, so that the pulsive noises are never outputted from the gate circuit.
FIG. 2 shows an arrangement of a prior art gate circuit configured to supply such an output. Reference numeral 1 denotes an input terminal in which an input signal S is entered, 2 designates an output terminal, 3 refers to an input terminal to which a control signal is applied, 4 to a low output impedance buffer amplifier, 5 to a switching circuit normally taking its on-position, 6 to a high input impedance buffer amplifier, 7 to a preceding value holding capacitor, and 12 to a noise detector detecting noise components in the input signal S and producing a control signal.
With this arrangement, an input signal entered in the input terminal 1 is applied to the switching circuit 5 via the low output impedance buffer amplifier 4. The switching circuit 5 is controlled by a control signal supplied from the input terminal 3 and maintains its on-position unless pulsive noises exist. Therefore, the signal entered in the input terminal 1 is supplied from the output terminal 2 via the switching circuit 5 and the high input impedance buffer amplifier 6.
When pulsive noises are detected, the input terminal 3 receives a control signal and changes the switching circuit 5 to its off-position, and the noises are removed. Meanwhile, a voltage charged in the preceding value holding capacitor 7 is outputted via the buffer amplifier 6. Since the buffer amplifier 6 has a high input impedance, the electric charge stored in the capacitor 7 is not discharged, and the terminal voltage of the capacitor 7 is maintained at a uniform level.
While the switching circuit 5 takes its on-position, distortions produced in the gate circuit are determined by properties of respective buffer amplifiers 4, 6 and the switching circuit 5. Distortions produced in semiconductive elements in the amplifiers is decreased to a significantly small value by using as the amplifiers negative feedback amplifiers in which the gain is 1.
However, it is difficult to restrain distortions produced in semiconductive elements in the switching circuit 5, the distortions occupy a large part of the distortions produced in the switching circuit 5. Particularly, when the frequency of a signal entered in the gate circuit 5 increases, the high band frequency characteristic is detracted by a low pass filter formed by resistance components of the switching circuit 5 and the capacitor 7.